In welded structures, fractures are mostly likely to occur in welded joints. There are several reasons. One is that welding defects that occur during welding become stress concentrators where fractures start. Another reason is that welding heat coarsens the microstructure of steel plates and, as a result, lowers the fracture toughness Kc that is used as a measure of brittle fracture resistance in welded joints.
In order to prevent deformation and stress concentration in welded joints, it is a basic requirement, in forming welded joints, to make the strength and hardness of the weld metal higher than those of the base metal. That is to say, welded joints are designed to have greater strength than the base metal.
Fracture toughness of welded joints is evaluated by a deep notch test that pulls, in the directions indicated by arrows, a test specimen having, as a norm, a 240 mm long notch 3 machined in assumedly the most weak part of welded joint in the middle of a 400 mm wide specimen 1 having a weld metal 2 at the center thereof, as shown in FIG. 4.
Conventionally fracture toughness of welded joints in steel plates for ship structures not more than 50 mm thick have been evaluated by this test and the performance and characteristics required of steel plates for ship construction have been considered.
Steel plates for ship construction having excellent brittle fracture and fatigue characteristics (TMCP steel plates) have been developed by considering the fracture toughness of welds (such as one disclosed in Japanese Unexamined Patent Publication No. 06-88161).
TMCP or other similar steel plates approximately 50 mm in thickness have been used for the construction of large tankers and container ships of not more than 6000 TEU. As construction needs for container ships larger than 6000 TEU have increased, steel plates 60 mm thick or more are being used.
While the upper limit of yield strength of steel plates for ship construction presently in use is approximately 390 MPa, thicker steel plates (such as those thicker than 50 mm) will be used as the size of container ships grows larger.
However, an excess steel plate thickness increase gives rise to various industrial problems, such as increases in welding man-hours, construction cost and the weight of container ships.